High-speed semiconductor lasers and modulators are essential components in today's fiber-optic communication systems. The rapid increase in internet traffic has demanded these optical components to be able to handle greater bit rates. Direct amplitude modulation by varying the bias current of the laser is the simplest method, without a need for a separate modulator. However, the directly modulated laser has fundamental speed limits, and will display transient oscillation at a frequency equal to its relaxation oscillation frequency. Wavelength chirp is another problem arising in directly modulated lasers. As the input drive current of a laser changes, so does the carrier density, hence refractive index, and therefore wavelength. The laser wavelength moves in opposite directions respectively as the pulse rises and falls. The higher the bit rate, the more the chirp begins to manifest itself as an effective widening of the laser linewidth. Due to chromatic dispersion in optical fibers, pulse spreading is more severe in the case of a wider laser linewidth, thereby limiting the transmission distance.
It is also highly desirable to have semiconductor lasers that is tunable or wavelength-switchable for dynamically reconfigurable networks. However, the phase change associated with the direct modulation often interferes with the wavelength tuning mechanism and the stability of the laser.
It is possible to keep the laser in continuous wave (CW) operation, and modulate it externally. This would eliminate the aforementioned problem of transient oscillation, and reduce the chirp, provided that the modulator suffers from less severe chirp than the laser. An electroabsorption modulator (EAM) is a viable option as an external modulator. Some of its advantages compared to other alternatives are: low cost, low drive voltages, small size, and the ability to be monolithically integrated with distributed feedback (DFB) or distributed Bragg reflector (DBR) lasers. An EAM is based on a very similar structure to the laser, with an active layer of a slightly different bandgap energy. Another difference is that it is operated in reverse bias. As the input stream of data bits alters the modulator reverse bias, the absorption coefficient of the modulator changes, thus varying the transmitted optical power.
Although the EAM improves the chirp performance considerably compared to direct modulation of the laser, the chirp problem remains due to refractive index change intrinsically associated with the modulation of absorption coefficient. More importantly, the modulator chirp is dynamic and changes with the actual drive voltage. Electro-absorption modulators now provide modulation capability up to about 10 Gb/s. At the moment it is not clear that electro-absorption modulators can reach higher speed (e.g. 40 Gb/s) without introducing considerable parasitic phase modulation.
Another possibility to modulate light is to use a Mach-Zehnder interferometer in a material showing strong electro-optic effect (such as LiNbO3). By applying a voltage the optical signal in each path is phase modulated as the optical path length is altered by the electric field. Combining the two paths with different phase modulation converts the phase modulation into intensity modulation. If the phase modulation is exactly equal in each path but different in sign, the modulator is chirp free, this means the output is only intensity modulated without parasitic phase or frequency modulation. However, such an external modulator is very expensive.
With the deployment of fiber-to-the premise (FTTP) technology for broadband access and the spread of dense wavelength division multiplexing (DWDM) in metro and local networks, low-cost semiconductor lasers and modulators have become more and more important. Fabry-Perot lasers are commonly used in access and enterprise networks, as well as in fiber channels for storage area networks. The dynamic wavelength stability of Fabry-Perot lasers has significant impact on the transmission distance, especially when multi-mode optical fiber is used for some of those applications. It is therefore highly desirable to have a high-speed low-cost modulator that causes little wavelength chirp or instability of the laser, not only for long-haul and metro applications, but also for access and enterprise networks.
It is an object of the present invention to provide a semiconductor laser monolithically integrated with a high-speed low-chirp modulator which features low-cost and fabrication simplicity similar to that of a directly modulated laser.